> Since Lojban has a six-vowel system, I would have thought that there
> would be a bit of latitude or room for allophonic variation in the
> vowels -- for example, to pronounce /o/ as [o] or [O], /a/ as [a] or
> [A], etc.

Lojbanists are surprisingly picky about their vowel sounds. As a
sloppy-vowelled 'merkin it's been frustrating to me, over the years,
to for instance record a whole story and then the only detailed
comment I get is a critque of some of my "y"s. :)
I believe that [O] for "o" and [A] for "a" are allowed variations, but
they're somewhat nonstandard. [A] I think I might hear occasionally,
but I don't recall ever hearing many [O]s (though I could be wrong,
I'm a terrible phonologist).
Lojban has a sense of two different levels of properness: There is a
range of allowable variation, within which [A] and [O] are surely
included, but there is also a shared vision of an ideal form, a one
proper way from which all else is deviation.
BTW all of my comments here are descriptive, not prescriptive. I
personally enjoy when people speak Lojban with different accents! I
always encourage people to speak Lojban "cilce" (wildly, untamed).
But Lojban has a long, proud tradition of pickiness. ;)
<3,
la stela selckiku